Mass Transfer of Hydrogen Sulfide at Turbulent Water Surface by Falling Drops or a Single Jet

Hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$) is a primary cause of odor and corrosion in sewer systems. In drop structures, falling wastewater impinges the surface of bottom water pool as a jet or numerous drops, triggering a high level of turbulence and thus enhancing the emission of ${\mathrm{H}}_2\mathrm{S}$ and the absorption of oxygen (${\mathrm{O}}_2$) at the pool surface. In this study, laboratory experiments were conducted to study the mass transfer at the pool surface with falling water drops or a single jet of water. In the test range of falling flow rate of $49–223\mathrm{mm}/\mathrm{h}$ and falling velocity of $3.1–5.2\mathrm{m}/\mathrm{s}$ [kinetic energy flux $(\mathrm{KEF})=0.11–0.80\mathrm{J}/({\mathrm{m}}^2·\mathrm{s})]$, the mass transfer coefficient $K_L$ for the pool surface was found to be $2.6–14.8\times {10}^{-5}\mathrm{m}/\mathrm{s}$ for ${\mathrm{H}}_2\mathrm{S}$. In addition, $K_L$ was found to be 76% larger when the pool surface was impinged by water drops than by a single jet. $K_L$ was $27\%-47\%$ larger in the half of the water surface directly receiving the drops or jet than the other half. The increase of water depth in the pool promoted the mass transfer, especially for the scenario of falling jet. Equations were proposed to predict $K_L$ under drops or jet. Moreover, $K_L$ for ${\mathrm{H}}_2\mathrm{S}$ and for ${\mathrm{O}}_2$ was found almost the same, indicating ${\mathrm{O}}_2$ can be a safe surrogate gas for ${\mathrm{H}}_2\mathrm{S}$. Finally, the research results were applied to estimate ${\mathrm{H}}_2\mathrm{S}$ emissions at the pool surface of a prototype drop structure with a falling height of up to 10 m, which showed that turbulent wastewater surface can enhance mass transfer over 100 times compared with a quiescent surface.